Mechanical analysis of a natural example of onland gravity gliding: The role of river incision and deposition
Résumé
Gravity gliding implies rigid translation of a body down a slope where displacements are parallel to a tilted detachment plane. Although large-scale gravity gliding is commonly observed offshore, under conditions of high fluid overpressure and abundant upslope sedimentary supply, its occurrence on land is debated. We investigate the mechanical feasibility of such a process as well as the role of fluvial incision and sedimentation down the slope in the initiation of the gliding. We use a two-dimensional (2-D) finite element model combined with a 2-D failure analysis approach. The numerical models simulate the deformation and provide quantitative estimates of the failure criteria at the head and toe of the overburden. Analytical solutions approximate the numerical results by taking into account the fluvial incision and sedimentation, the internal friction angle, and the thickness and length of the overburden. Our models are based on a field example in the Andean foothills of Argentina, where gravity gliding of a 1000 m thick section is suspected above a crustal-scale anticline. The incision and sedimentation reduce and strengthen, respectively, the downslope resistance to contractional failure. The critical slope at which the gliding is initiated is reduced by fluvial incision and increased by sedimentation. We show that tectonic uplift may lead to large-scale gravity gliding on land where the overburden thickness is less than 2000 m. Incision facilitates and localizes the frontal shortening. Incision greater than 1000 m may trigger gliding for overburden up to 4000 m thick, while sedimentation thicker than 1000 m inhibits gliding. These results show that thin-skinned onland gravity gliding could be common in tectonically active regions where incision is important.
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